The present invention relates to monolithic chip capacitors, especially chip capacitors adapted to be connected directly to circuit boards as opposed to capacitors having flexible leads extending therefrom.
Increasing use is being made of chip capacitors because of their compactness and inherent reliability. Such capacitors are conventionally provided with two or more termination portions at their end edges and are mounted directly to conductive lands on the surface of a substrate formed of alumina, epoxy-filled fiber glass or the like. The terminations are placed in direct contact with the lands and are mechanically and electrically connected thereto by solder or a conductive epoxy bonding material.
During the fabrication of electronic circuits on the printed circuit board and/or during the use of such circuit, temperature cycling is encountered. Because of the mismatch in thermal expansion coefficients between the materials of the capacitor chip and the substrate or printed circuit board on which the chip is mounted, fracturing can occur in the chip body or at the interface between the termination and the capacitor electrodes.
To avoid such failure of discrete monolithic capacitors, attempts have been made to introduce a compliant connection at the interface between the capacitor termination and the circuit board such that the differential shrinkage and expansion of the components will not exert excessive stresses on the fragile elements of the device but rather, will be absorbed by flexure of the compliant connection. One such compliant member comprises a tab which, like the lead wire on a conventional capacitor, absorbs the stress introduced by differential expansion of components. Such a tab increases the cost of the capacitor chip by virtue of the presence of an additional part, together with the operations necessary to apply that part to the chip.
Another solution to the problem of capacitor chip failure is to form the chip in such a manner that the chip terminations are connected to the substrate by columns of solder, the dimensions of which render the solder connections compliant. A capacitor chip of this type is disclosed in U.S. Pat. No. 4,297,773. The capacitor chip is provided with support means such as a platform or a plurality of ridges which space the conductive terminations from the conductive lands on the substrate when the chip is positioned on the substrate. The distance between the substrate and the bottom of the capacitor chip in the region of the conductive terminations is said to be at least 5 mils to insure that a substantial expanse or vertical column of solder will exist between the conductive terminations of the capacitor and the substrate lands. There is thus provided a solder connection of sufficient length to provide a relatively high compliance connection between the capacitor and the conductive lands of the substrate thereby causing mechanically generated stress to be absorbed or compensated within the solder body rather than being transmitted to the capacitor at sufficiently high levels to cause capacitor failure or change in value. Because of the shape of that type of capacitor, special equipment is required for its manufacture. Also, special equipment is required for the placement of such capacitors on substrates since only one side of the capacitor can face the substrate.